The use of fiber optic technology has greatly increased in recent years. These fibers can transmit signals in many ranges of the electromagnetic spectrum, and have found wide use in communications, remote sensing, imaging, and lasers.
Of particular interest are glass fibers for fiber optic transmission in the infrared portion of the spectrum. Infrared-transmitting fiber optic technology can be used in Navy/DOD applications such as remote chemical sensor systems and sensors for use in cleanup of DOD facilities. Other important military applications of infrared-transmitting optical fibers provide superior aircraft survivability by their use in aircraft protection systems against heat-seeking missiles and laser threat warning systems. Still other applications of infrared-transmitting optical fibers include their use in high-energy infrared power delivery systems such as those using CO (5.4 μm) and CO2 (10.6 μm) lasers.
In addition, infrared-transmitting optical fibers are used in a myriad of other military and civilian applications. These applications include sensors for detection of contaminants in soil or groundwater, monitoring of environmental pollution, or application in other civil/industrial processes; optical fibers used in Raman amplifiers; photonic band gap fibers; and optical ultra-fast switches for telecommunications. Infrared-transmitting fibers also have important medical uses, such as in surgery and tissue diagnostics.
Thus, there has been an increased need for high quality infrared-transmitting optical fibers. One type of optical fibers that have seen significant use in recent years are fibers made using chalcogenide glass. Infrared-transmitting chalcogenide glasses and optical fibers made therefrom can be used for numerous applications involving infrared transmissions, including thermal imaging, temperature monitoring, and medical applications.
Chalcogenide glasses are made from mixtures of the chalcogen elements such as sulfur, selenium, and tellurium, which have two-folded coordination. Conventional arsenic selenide (As—Se) glass can have has a transmission range from 1 to 10 μm. However, such conventional glass tends to crystallize during reheating of the glass for fiber drawing. See M. F. Churbanov, et al., “Flow of molten arsenic selenide in a cylindrical channel,” Inorganic Materials, Vol. 39 No. 1, pp. 77-81, 2003. The presence of such crystals increases instability of the glass and can contribute to signal loss, limiting the usefulness of such glass for optical fibers.
The addition of network formers such as germanium or arsenic establishes cross-linking and facilitates stable glass formation. Depending on their composition, chalcogenide glass optical fibers having germanium and/or arsenic constituents can transmit infrared signals in a wider range than conventional As—Se glasses, i.e., from between about 1 to 12 μm. Tellurium also may be added to As—Se glasses to extend the long wavelength transmission.
Conventional chalcogenide glasses having germanium and tellurium as constituents, however, contain these elements in high amounts. For example, U.S. Pat. No. 4,908,053 to Nishii et. al. describes an As—Se glass having additional amounts of germanium and tellurium. The Ge—As—Se—Te glass described in Nishii et al. contains a high germanium (25 mol %) and high tellurium (30 mol %) concentration. Tikhomirov et al. has also published work regarding Ge—As—Se—Te glasses having 15 mol % germanium and up to 61 mol % tellurium. See V. K. Tikhomirov, et al., “Glass-formation in the Te-enriched part of the quaternary Ge—As—Se—Te system and its implication for mid-IR fibres,” submitted to Infrared Physics and Technology, March 2004.
The high tellurium concentration in the glasses described by Nishii and Tikhomirov can have significant drawbacks, however, which can limit the usefulness of such glasses for optical fibers. A high tellurium content shifts the electronic edge of the optical fiber to longer wavelengths and makes it impossible to use these glasses for applications at shorter wavelengths, particularly at 1.55 μm, which is an important wavelength for telecommunications applications. The high tellurium content in these glasses also makes the fibers more weak and fragile, further limiting their use in many applications. In addition, like conventional As—Se glasses, glasses having a high tellurium content are prone to crystallization as they are heated above the glass transition temperature Tg, which makes it difficult to make low-loss fibers, since the presence of crystals in the fibers contributes to signal loss. Moreover, as described by V. Q. Nguyen, et al., the high tellurium concentration increases the free carrier absorption loss at temperatures greater than 22° C., which puts a limit on the practical applications of the fibers since the temperature may not be constant. See V. Q. Nguyen, et al. “Very large temperature-induced absorptive-loss in high Te-containing chalcogenide fibers,” J. Lightwave Technology, vol. 18, no. 10, 1395-1401, October 2000. All of these aspects limit the usefulness of conventional Te-containing chalcogenide glasses as optical fibers.
Because of many potential applications in the mid-infrared range, there is a thus need to develop a new thermally stable glass that avoids crystallization at temperatures above the glass transition temperature Tg and during fiber drawing or other re-shaping of the glass above Tg and that can be used in optical fibers for transmissions from 1-10 μm with low signal loss.